1. Field of the Invention
The present invention relates to a scientific instrument capable of performing conventional thermogravimetric analysis. The instrument is also capable of measuring the mass of an unknown sample in ambient atmosphere. Further, the invention provides a heretofore unknown method of measuring the continuous mass change of a solid subjected to high sweep gas velocities. Further, the invention will perform continuous mass change analysis with both high sweep gas velocities and elevated temperatures.
Conventional thermogravimetric analysis techniques are subject to fluctuating and unstable weight readings as the sweep gas flow past the solid being analyzed is increased. The fluctuations not only reduce the accuracy of measurement at low gas flow rates, but may also grow very rapidly, totally invalidating thermogravimetric analysis weight readings even at moderate sweep gas flow rates. J. M. Forgac and J. C. Angus, in "A Pressurized Thermobalance Apparatus For Use at Extreme Conditions," (Industrial and Engineering Chemistry Fundamentals, Volume 18, No. 4, Page 416 (1979)) reported that the weight reading became unstable due to a natural convection current induced by the temperature difference between the gas and the solid.
This instability severely limits the application of thermogravimetric analysis techniques in the kinetic study of fast gas-solid reactions at elevated pressures and temperatures because:
(a) a high sweep gas rate, required to enhance the contact between gas and solid and thereby determine the reaction kinetics (exclusive of external heat and mass transfer resistances) generates intense flow turbulences which cause instability of the weight measurement;
(b) the flow turbulence generated increases as the gas is compressed to elevated pressures. The compressed gas then exerts an increased impact on the suspended solid;
(c) the temperature gradient developed about the solid generates a free convection current which adds to the flow turbulence.
The stability problems originate from the simple fact that gravity is a very weak force field and is easily disturbed by flow turbulences. It is generally considered unavoidable in conventional thermogravimetric analysis techniques. Due to the stability limitation of the conventional gravimetric mass measurement, the sweep gas velocity in conventional thermogravimetric reactors is very restricted and is seldom allowed to exceed 0.1 m/s. In this range of sweep gas velocity, the gas-solid mass and heat transfer rates are very low, Nu=2 as predicted by the Froessling equation, and the reaction often proceeds in the presence of significant temperature and composition gradients. The limitation on the sweep gas velocity is most pronounced at high pressures (high gas densities) and high temperatures (thermal disturbances) typical of industrial gas-solid reaction conditions.
The field of use for the present invention includes not only its application as a scientific instrument in the measurement of mass, but also in the duplication of measurements obtained in conventional thermogravimetric analysis. It also makes possible the measurement of the mass change of a solid subjected to high sweep gas velocities at elevated temperatures and pressures. Many industrially important gas-solid reactions are conducted at elevated pressures and temperatures such as those in coal gasification, coal combustion, oil shale retorting, dolomite sulfation, biomass pyrolysis, and mineral conversions.
Thermogravimetric analysis has rarely been used for liquid-solid contact systems because of the instability problems. The strong centrifugal force field of the present invention however makes possible the measurement of the mass and its change of a solid immersed in liquid. The present invention therefore also provides useful experimental means for the kinetic study of liquid-solid reactions such as those in coal liquification, leaching of minerals and ores, and purification of polluted water by activated carbon.
2. Discussion of the Prior Art
A conventional thermogravimetric analysis is disclosed in U.S. Pat. No. 3,973,636 which issued to Hiroshi Uchida on Aug. 10, 1976. This device is essentially a balance beam having a known reference material applied to one end of the balance beam and a test sample applied to the other end. The known reference material and the test sample are then subjected to high temperatures while any change in mass of the test sample is detected by an electromagnetic pickup device.
A general summary of thermogravimetric techniques may be found in "An Introduction to Thermogravimetry" by C. J. Keattch, FRIC published by Heyden and Son, Ltd. in cooperation with Sadtler Research Laboratories Inc., Page 1-14, 1969).
U.S. Pat. No. 3,812,924 to Fletcher et al. on May 28, 1974 discloses a device for monitoring a change in mass in varying environments. This device is a cantilever beam device using a strain gauge as the transducer for reflecting the change in mass of the sample.
The foregoing patents and the book excerpt describe conventional systems for thermogravimetric mass analysis. As discussed above, conventional systems are not capable of measuring the continuous mass change of a test solid under high sweep gas velocities.
The two most widely used laboratory reactors for fluid-solid reaction studies are described on pages 535 to 537 of the textbook entitled "Chemical Engineering Kinetics" 3rd Edition, authored by J. M. Smith, published by McGraw Hill Book Company in 1981. These reactors provide uniform fluid conditions and high fluid-solid contacting velocities. However, they are not capable of measuring the changing mass of the reacting solid. This textbook does describe on Page 640-642 a prior attempt to obtain uniform fluid conditions at high fluid-solid contacting velocities in conventional thermogravimetric analysis. This technique has had some success with an extremely big and heavy single suspended particle with a diameter greater than one inch. It is not capable of handling ordinary solid particles which are much smaller than one inch. The device illustrated, on page 641 (FIG. 14-2) is a stirred-tank single-pellet reactor used for the kinetic study of hydrofluorination of uranium dioxide.
Basket-type mixed reactors are also used in gas-solid contact systems. Such a device is disclosed in pages 485-487 of "Chemical Reaction Engineering", 2nd Edition by Octave Levenspiel, published by John Wiley and Sons, Inc. in 1972.
The fluid-solid contact devices disclosed above are not capable of measuring continuous mass changes in test samples under high sweep gas rate velocities. Even in the "stirred-tank single pellet reactor" the stirring speed is very restricted due to the stability limitation.
The present invention uses centrifugal force to amplify the weight of the sample to be tested. In conventional thermogravimetric analysis, the weight of the sample is determined by the gravitational pull on the mass of the sample. In the present invention, the weight of the sample is greatly amplified by centrifugal force.
U.S. Pat. No. 2,826,079 which issued to M. L. Kuder et al. on Mar. 11, 1958 discloses an automatic coin weighing machine which in FIG. 5 discloses a standard reference weight indicated by the numeral 4, and a coin to be sampled indicated by the numeral 5. If the coin is a counterfeit coin the difference in weight between the standard reference weight and the counterfeit coin will displace the center of mass slightly from the geometric center of the wheel. The apparatus then detects the displacement with an electronic mutual inductance micrometer. This patent teaches the concept of magnifying the apparent mass of the sample to be tested by nearly 500 times. This magnifies the small weight differential between the standard reference weight and the counterfeit coin 500 fold.
U.S. Pat. No. 2,814,944 to R. E. Brown issued on Dec. 3, 1957 discloses a centrifugal testing apparatus for instruments. This device has some structural similarities to the structures employed in one of the embodiments of applicant's invention. In this device, a pair of outwardly extending support arms rotate about a center axis. Each of the outwardly extending support arms carries a basket. One of the baskets is loaded with the instruments to be tested, the other basket is loaded with the appropriate counterweights to balance out the centrifuge. Dynamic unbalance above a predetermined tolerance is detected automatically and corrected by a servo-motor within the mechanism. If the dynamic imbalance is too high, the mechanism is shut down completely.
It should be noted that neither of the devices illustrated in the Brown '944 patent nor the Kuter et al. '079 patent are capable of measuring a mass change in a coin or in the instrument. In addition, they are not capable of measuring a mass change under extreme thermal conditions, or under high sweep gas velocities and elevated temperatures and pressures.